Tagging of antialiased images

Details Tagging of antialiased images Tagging of antialiased images

1998-03-23

2001-06-05

Lee, Thomas D. (Department: 2724)

Image analysis

Image enhancement or restoration

Edge or contour enhancement

C382S266000

Reexamination Certificate

active

06243499

ABSTRACT:

This invention relates generally to a method and apparatus to enable the printing of antialiased images and other image structures with gray borders, and more particularly to an edge-identification method, which can be implemented in logic, for generating antialiased rendering tags and rendering signals within an architecture designed for the printing antialiased text or line regions.
CROSS REFERENCE
The following related applications are hereby incorporated by reference for their teachings:
“METHOD FOR GENERATING RENDERING TAGS TO FACILITATE THE PRINTING OF ANTIALIASED IMAGES,” P. Crean et al., Application Ser. No. 09/046,232, filed concurrently herewith, now U.S. Pat. No. 6,144,461;
“A METHOD OF ENCODING HIGH RESOLUTION EDGE POSITION INFORMATION IN CONTINUOUS TONE IMAGE INFORMATION,” N. Zeck et al., Application Ser. No. 09/046,231, filed concurrently herewith, now U.S. Pat. No. 6,020,979;
“TEXT QUALITY ENHANCEMENT VIA RESOLUTION ENHANCEMENT TECHNIQUE BASED ON SEPARATING JAGGEDNESS DETECTION AND FILTERING,” R. Eschbach, Application Ser. No. 08/937,406, filed Sep. 25, 1997, now U.S. Pat. No. 5,956,470;
“METHOD TO ENABLE THE RECOGNITION AND RENDERING OF ANTIALIASED IMAGES,” R. Loce et al., Application Ser. No. 09/046,414, filed concurrently herewith, now U.S. Pat. No. 6,167,166; and
“MEMORY-EFFICIENT TAGGING OF ANTIALIASED IMAGES,” S. Harrington et al., Application Ser. No. 09/046,426 filed concurrently herewith, now U.S. Pat. No. 6,137,918.
BACKGROUND AND SUMMARY OF THE INVENTION
Antialiasing in the context of digitizing line art and certain graphical image structures is best known as a method of using intermediate levels of intensity to achieve subpixel position of edges for several reasons including reduction or elimination of jaggies on the edges of lines and polygons, including text. As used herein the term antialiased is intended to refer to those segments or regions of an image that are effected by an antialiasing operation applied to the image (e.g. an image processing operation or a physical process resulting in gray pixels along the edges of line art or text). Jaggies are, primarily visible at the edges of sloped lines approaching horizontal or vertical. The term antialiasing suggests an analog term aliasing; normally representing the presence of low frequencies resulting from sampling high frequency signals at too low a sampling rate.
Consider a near-vertical (or near-horizontal) line segment. To be perfectly reproduced in a printed media, the phase, which represents the location of the edge, must continuously vary along the length of a segment. Due to the inherent sampling of a bi-level display or printed output, the phase exhibits jump discontinuities. Thus, this form of aliasing artifact, leads to an induced jagged appearance where the structures are referred to as jaggies. Within a sampled image any graphical object is eventually approximated as a polygon or collection of polygons. These polygons have straight edges some of which will exhibit aliasing (jaggies and other placement defects).
FIG. 1
for example shows aliasing in two dimensions. When the triangle on the top of
FIG. 1
is rasterized, the edges are aliased as reproduced in the triangle shown at the bottom of FIG.
1
. In particular, the position along the bottom edge should move up slightly from column to column as one looks from left to right in the image at the bottom of FIG.
1
. However, the position is quantized, as illustrated, producing the jagged appearance along the bottom of the triangle. Visibility of the anti-aliased image artifacts is increased by the regular nature of the jaggies, again a result of sampling.
Consider the following systems and their capability, or incapability, to utilize antialiased pixels. Xerox's Docucolor 40, for example, employs a high frequency analog line screen to render antialiased pixels, but that is not an option for some products or marked segments. When conventional screens (e.g., approximately equal to 130-150 CPI dots) are employed in a rendering module, antialiased pixels are halftoned and printed, resulting in objectionable halftone dots positioned along character edges. Hyperacuity printing techniques, for example those described by Curry, et al. (U.S. Pat. No. 5,138,339 and U.S. Pat. No. 5,485,289) can provide rendering for antialiased pixels that is compatible with simultaneously printing dot screen halftones in enhanced line art. However, these techniques require the use of tags to identify the antialiased pixels as antialiased line art. In the preferred embodiments described with respect to the present invention the rendering architecture distinguishes text/line art from contone images to appropriately treat both image types. As will be described herein an algorithm or method may be employed in a rendering module or in other components of the rendering device to convert gray antialiased pixels to a form suitable for xerographic printing.
Antialiased images can be generated by capturing the image at a resolution greater than the final or desired output resolution, then reducing the resolution of the image by sub-sampling using an averaging process. A major benefit of antialiased images is that high contrast, saturated objects are surrounded with pixels possessing intermediate values that visually suggest the true, higher resolution position of object edges.
For example, in binary printing systems, such as many xerographic or ink jet systems that use a halftoning process to simulate continuous tone images, these antialiased edge pixels should be rendered with a very high frequency cell, ideally one having the resolution of the final output image. If the standard system halftone dot were to be used, the antialiased edges would be serrated or jagged at the standard halftone frequency. This rendering would reduce or even negate any value obtained through antialiasing. The use of a very high frequency screen over the entire antialiased image renders the antialiased pixel properly, but tends to sharpen the tonal curve and provoke print quality defects in the overall image.
Antialiasing produces image structures with gray borders or boundaries. Gray borders can also be present in an image due to the nature of a particular image structure. For instance, a gray character on a white background and a white character on a gray background also possess gray borders. As is the case for antialiased edge pixels, it is generally desirable to render the borders for gray objects with a high frequency screen, while is may be desired to render the interior of the gray stroke with a lower frequency screen chosen for attributes other than edge rendition. Through an optimized use of thresholds, these border pixels can be tagged, and subsequently optimally rendered, in a similar manner to border pixels that are gray due to antialiasing. This form of selective rendering will produce a high frequency edge outline at the gray-to-white border (note the border can be also be between two gray levels given the use for adaptive thresholds). Throughout the present discussion both types of border pixels (from gray objects and from antialiased objects) will sometimes be referred to simply as antialiased pixels.
Hence, the present invention is directed to a method for detecting antialiased image regions and creating rendering tags within an architecture designed for the rendering of antialiased text or line regions, thereby enabling the antialiased pixels to be rendered in a manner distinguishable from that applied to continuous tone portions of an image.
Heretofore, a number of patents and publications have disclosed information relevant to antialiasing, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,646,751 to Motamed et al., issued Jul. 8, 1997, teaches a method for improving the speed of a color conversion operation using pixel tagging.
In “A Comparison of Antialiasing Techniques,” IEEE CG&A, Vol. 1, No. 1, January 1981, pp. 40-48, F. Crow teaches that prefiltering is a computationally effective technique for antialiasing.
U.S. Pat. No. 5,

Affiliated with

Also associated with

Rating

Tagging of antialiased images does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Tagging of antialiased images, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tagging of antialiased images will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2521868